The present invention relates to a method of pretreating steel-hybrid printed circuit boards, particularly those which are intended to be provided with surface mounted electronic components, before coatings that are to be burnt in or fired are applied.
It is known to produce steel-hybrid circuit boards of this type in that a carrier plate in the form of a steel-hybrid base plate is covered with various electrically conductive and electrically insulating coatings, respectively, which after their respective application are fired in a conventional kiln.
One problem encountered in the production of such circuit boards is that at the customary firing temperatures in the kiln, the differences in coefficients of thermal expansion of the carrier material (here the steel-hybrid or substrate) and of the respective coating produce different thermal stresses. As the person skilled in the art is aware, in the past this has always resulted in warping of the fired circuit board.
In the past, and as a remedy and solution of the above-described problem, a very thin coating has been applied to a rather thick steel-hybrid carrier. Thin coatings, however, exhibit adhesion problems and frequently result in undue electrical breakdown voltages. Moreover, thick steel-hybrid carrier plates have an undesirably high weight, frequently increase the time required for the entire manufacturing process, and are expensive. Thus, to obtain a circuit board that is not noticeably warped it has been necessary to revert, for example, to a carrier plate thickness of about 4-6 mm for a coating thickness of 150 .mu.m.
Because of the problems described above, so-called multi-layer steel-hybrid circuit boards having planar conductor paths in different layers have not yet been able to be produced.